[Mlir-commits] [mlir] [mlir][amdgpu] Shared memory access optimization pass (PR #75627)

[llvmlistbot at llvm.org]

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+//===- OptimizeSharedMemory.cpp - MLIR AMDGPU pass implementation ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements transforms to optimize accesses to shared memory.
+// It is inspired by
+// https://github.com/llvm/llvm-project/blob/main/mlir/lib/Dialect/NVGPU/Transforms/OptimizeSharedMemory.cpp
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/AMDGPU/Transforms/Passes.h"
+
+#include "mlir/Dialect/AMDGPU/IR/AMDGPUDialect.h"
+#include "mlir/Dialect/AMDGPU/Transforms/Transforms.h"
+#include "mlir/Dialect/AMDGPU/Transforms/Utils.h"
+#include "mlir/Dialect/Arith/IR/Arith.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/Dialect/GPU/IR/GPUDialect.h"
+#include "mlir/Dialect/MemRef/IR/MemRef.h"
+#include "mlir/Dialect/Vector/IR/VectorOps.h"
+#include "mlir/Interfaces/SideEffectInterfaces.h"
+#include "mlir/Support/LogicalResult.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/MathExtras.h"
+
+namespace mlir {
+namespace amdgpu {
+#define GEN_PASS_DEF_OPTIMIZESHAREDMEMORY
+#include "mlir/Dialect/AMDGPU/Transforms/Passes.h.inc"
+} // namespace amdgpu
+} // namespace mlir
+
+using namespace mlir;
+using namespace mlir::amdgpu;
+
+/// The size of a shared memory line according to AMD documentation.
+/// https://www.amd.com/content/dam/amd/en/documents/instinct-tech-docs/instruction-set-architectures/instinct-mi200-cdna2-instruction-set-architecture.pdf
+constexpr int64_t kSharedMemoryLineSizeBytes = 64;
+/// We optimize for 64bit accesses, but this can be made an argument in the
+/// future.
+constexpr int64_t kDefaultVectorSizeBits = 64;
+
+/// Uses `srcIndexValue` to permute `tgtIndexValue` via
+/// `result = xor(floordiv(srcIdxVal,permuteEveryN),
+///               floordiv(tgtIdxVal,vectorSize)))
+///            + tgtIdxVal % vectorSize`
+/// This is done using an optimized sequence of `arith` operations.
+static Value permuteVectorOffset(OpBuilder &b, Location loc,
+                                 ArrayRef<Value> indices, MemRefType memrefTy,
+                                 int64_t srcDim, int64_t tgtDim) {
+  // Adjust the src index to change how often the permutation changes
+  // if necessary.
+  Value src = indices[srcDim];
+
+  // We only want to permute every N iterations of the target dim where N is
+  // ceil(sharedMemoryLineSizeBytes / dimSizeBytes(tgtDim)).
+  const int64_t permuteEveryN = std::max<int64_t>(
+      1, kSharedMemoryLineSizeBytes / ((memrefTy.getDimSize(tgtDim) *
+                                        memrefTy.getElementTypeBitWidth()) /
+                                       8));
+
+  // clang-format off
+  // Index bit representation (b0 = least significant bit) for dim(1)
+  // of a `memref<?x?xDT>` is as follows:
+  // N := log2(128/elementSizeBits)
+  // M := log2(dimSize(1))
+  // then
+  // bits[0:N] = sub-vector element offset
+  // bits[N:M] = vector index
+  // clang-format on
+  int64_t n =
+      llvm::Log2_64(kDefaultVectorSizeBits / memrefTy.getElementTypeBitWidth());
+  int64_t m = llvm::Log2_64(memrefTy.getDimSize(tgtDim));
+
+  // Capture bits[0:(M-N)] of src by first creating a (M-N) mask.
+  int64_t mask = (1LL << (m - n)) - 1;
+  if (permuteEveryN > 1)
+    mask = mask << llvm::Log2_64(permuteEveryN);
+  Value srcBits = b.create<arith::ConstantIndexOp>(loc, mask);
+  srcBits = b.create<arith::AndIOp>(loc, src, srcBits);
+
+  // Use the src bits to permute the target bits b[N:M] containing the
+  // vector offset.
+  if (permuteEveryN > 1) {
+    int64_t shlBits = n - llvm::Log2_64(permuteEveryN);
+    if (shlBits > 0) {
+      Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, shlBits);
+      srcBits = b.createOrFold<arith::ShLIOp>(loc, srcBits, finalShiftVal);
+    } else if (shlBits < 0) {
+      Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, -1 * shlBits);
+      srcBits = b.createOrFold<arith::ShRUIOp>(loc, srcBits, finalShiftVal);
+    }
+  } else {
+    Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, n);
+    srcBits = b.createOrFold<arith::ShLIOp>(loc, srcBits, finalShiftVal);
+  }
+
+  Value permutedVectorIdx =
+      b.create<arith::XOrIOp>(loc, indices[tgtDim], srcBits);
+  return permutedVectorIdx;
+}
+
+static void transformIndices(OpBuilder &builder, Location loc,
+                             SmallVector<Value, 4> &indices,
+                             MemRefType memrefTy, int64_t srcDim,
+                             int64_t tgtDim) {
+  indices[tgtDim] =
+      permuteVectorOffset(builder, loc, indices, memrefTy, srcDim, tgtDim);
+}
+
+/// Return all operations within `parentOp` that read from or write to
+/// `shmMemRef`.
+static LogicalResult
+getShmReadAndWriteOps(Operation *parentOp, Value shmMemRef,
+                      SmallVector<Operation *, 16> &readOps,
+                      SmallVector<Operation *, 16> &writeOps) {
+  parentOp->walk([&](Operation *op) {
+    MemoryEffectOpInterface iface = dyn_cast<MemoryEffectOpInterface>(op);
+    if (!iface)
+      return;
+    std::optional<MemoryEffects::EffectInstance> effect =
+        iface.getEffectOnValue<MemoryEffects::Read>(shmMemRef);
+    if (effect) {
+      readOps.push_back(op);
+      return;
+    }
+    effect = iface.getEffectOnValue<MemoryEffects::Write>(shmMemRef);
+    if (effect)
+      writeOps.push_back(op);
+  });
+
+  // Restrict to a supported set of ops. We also require at least 2D access,
+  // although this could be relaxed.
+  if (llvm::any_of(readOps, [](Operation *op) {
+        return !isa<memref::LoadOp, vector::LoadOp, vector::TransferReadOp>(
+                   op) ||
+               amdgpu::getIndices(op).size() < 2;
+      }))
+    return failure();
+  if (llvm::any_of(writeOps, [](Operation *op) {
+        return !isa<memref::StoreOp, vector::StoreOp, vector::TransferWriteOp>(
+                   op) ||
+               amdgpu::getIndices(op).size() < 2;
+      }))
+    return failure();
+
+  return success();
+}
+
+mlir::LogicalResult
+mlir::amdgpu::optimizeSharedMemoryReadsAndWrites(Operation *parentOp,
+                                                 Value memrefValue) {
+  auto memRefType = dyn_cast<MemRefType>(memrefValue.getType());
+  if (!memRefType ||
+      !amdgpu::AMDGPUDialect::hasSharedMemoryAddressSpace(memRefType))
+    return failure();
+
+  // Abort if the given value has any sub-views; we do not do any alias
+  // analysis.
+  bool hasSubView = false;
+  parentOp->walk([&](memref::SubViewOp subView) { hasSubView = true; });
+  if (hasSubView)
+    return failure();
+
+  // Check if this is necessary given the assumption of 128b accesses:
+  // If dim[rank-1] is small enough to fit 8 rows in a 128B line.
+  const int64_t rowSize = memRefType.getDimSize(memRefType.getRank() - 1);
+  const int64_t rowsPerLine =
+      (8 * kSharedMemoryLineSizeBytes / memRefType.getElementTypeBitWidth()) /
+      rowSize;
+  const int64_t threadGroupSize =
+      1LL << (7 - llvm::Log2_64(kDefaultVectorSizeBits / 8));
+  if (rowsPerLine >= threadGroupSize)
+    return failure();
+
+  // Get sets of operations within the function that read/write to shared
+  // memory.
+  SmallVector<Operation *, 16> shmReadOps;
+  SmallVector<Operation *, 16> shmWriteOps;
+  if (failed(getShmReadAndWriteOps(parentOp, memrefValue, shmReadOps,
+                                   shmWriteOps)))
+    return failure();
+
+  if (shmReadOps.empty() || shmWriteOps.empty())
+    return failure();
+
+  OpBuilder builder(parentOp->getContext());
+
+  int64_t tgtDim = memRefType.getRank() - 1;
+  int64_t srcDim = memRefType.getRank() - 2;
+
+  // Transform indices for the ops writing to shared memory.
+  while (!shmWriteOps.empty()) {
+    Operation *shmWriteOp = shmWriteOps.pop_back_val();
+    builder.setInsertionPoint(shmWriteOp);
+
+    auto indices = amdgpu::getIndices(shmWriteOp);
+    SmallVector<Value, 4> transformedIndices(indices.begin(), indices.end());
+    transformIndices(builder, shmWriteOp->getLoc(), transformedIndices,
+                     memRefType, srcDim, tgtDim);
+    amdgpu::setIndices(shmWriteOp, transformedIndices);
+  }
+
+  // Transform indices for the ops reading from shared memory.
+  while (!shmReadOps.empty()) {
+    Operation *shmReadOp = shmReadOps.pop_back_val();
+    builder.setInsertionPoint(shmReadOp);
+
+    auto indices = amdgpu::getIndices(shmReadOp);
+    SmallVector<Value, 4> transformedIndices(indices.begin(), indices.end());
+    transformIndices(builder, shmReadOp->getLoc(), transformedIndices,
+                     memRefType, srcDim, tgtDim);
+    amdgpu::setIndices(shmReadOp, transformedIndices);
+  }
+
+  return success();
+}
+
+namespace {
+class OptimizeSharedMemoryPass
----------------
erman-gurses wrote:

Changed, thanks.

https://github.com/llvm/llvm-project/pull/75627